1. Field of the Invention
The present invention relates to a control device for an internal combustion engine for controlling the operation timing of an intake valve or an exhaust valve in the internal combustion engine.
2. Description of the Related Art
Up to now, there has been known a valve timing control device for an internal combustion engine, which changes a phase angle of a cam shaft with respect to a crank shaft in the internal combustion engine to thereby change a valve switching timing of the intake valve or the exhaust valve (for example, refer to JP 2001-234765 A).
The valve timing control device of this type is provided with a crank angle sensor for outputting a crank angle signal at a reference rotation position of the crank shaft, and a cam angle sensor for outputting a cam angle signal at a reference rotation position of the cam shaft. A real phase angle of the cam shaft is detected based on detection signals of the crank angle sensor and the cam angle sensor, and a phase angle feedback control is conducted so that the real phase angle coincides with a target phase angle that is set based on an operation state of the internal combustion engine.
The phase angle of the cam shaft with respect to the crank shaft is changed by a cam shaft phase variable mechanism in which the hydraulic supply is controlled by a hydraulically controlled solenoid valve. The hydraulically controlled solenoid valve is constructed of a duty solenoid valve, and a supply voltage to the solenoid is controlled in duty ratio to control a current value. A hydraulic pressure is selectively supplied to an advance chamber or a delay chamber of the cam shaft phase variable mechanism to change the cam shaft to the advance side or the delay side. Further, when the duty ratio is a retention duty value in the vicinity of the center, the hydraulically controlled solenoid valve closes the advance chamber and the delay chamber at the same time. Then, the hydraulically controlled solenoid valve is controlled to a neutral position where the supply of the hydraulic pressures is cut off at the same time. As a result, the phase angle of the cam shaft is retained.
In order to compensate a variation in the retention duty value with which the hydraulically controlled solenoid valve is set at the neutral position due to a tolerance of the hydraulically controlled solenoid valve or a variation with time, there have been known a method of learning the retention duty value and a method of storing the learned value in a backup RAM. There has also been known a method of using a fixed value that has been stored in a ROM in advance as an initial value when the retention duty value is not learned at all or when the learned value is lost upon, for example, turning off of a battery (disconnection of a battery terminal).
However, because the fixed value of the retention duty set as described above varies in the tolerance and also changes with time, the fixed value may not naturally coincide with the learned value that compensates those variations. For that reason, in the case where such an inconsistency occurs therebetween, the use of the fixed value of the retention duty value as the initial value when the battery is in an off state causes displacement of an actual position of the hydraulically controlled solenoid valve in the retention state from the original neutral position. As a result, the subsequent controllability of the cam phase control is also deteriorated.
In particular, in the case where the inconsistency occurs at the advance side, and a target phase angle is set to the advance side where a valve overlap of the intake valve and the exhaust value is originally large, it is also known that the valve overlap becomes excessive, and an internal exhaust gas recirculation volume (EGR volume) is resultantly excessive, which may deteriorate the combustion quality.
For that reason, in the valve timing control device for an internal combustion as disclosed in JP 2001-234765 A, the learned retention duty value is set as an initial value of an integral term of the feedback control, and in the case where the learning of the retention duty has not yet been completed, the target phase angle is limited.
However, in the valve timing control device for an internal combustion engine disclosed in JP 2001-234765 A, the retention duty fluctuates due to a change in resistance value of a hydraulically controlled solenoid coil or a change in battery voltage, which is attributable to a change in oil temperature. As a result, in the case where a temperature of the hydraulically controlled solenoid coil and the battery voltage at the time of learning the retention duty are different from a temperature and a voltage at the time of setting the learned retention duty value to the initial value of the integral term at the time of starting the phase angle feedback control, the actual value of the retention duty value and the learned value are different from each other.
In the above case, when the learned retention duty value is set to the initial value of the integral term at the time of starting the phase angle feedback control after the internal combustion engine starts, the real position in the retention state of the hydraulically controlled solenoid valve is deviated from the original neutral position. In particular, in the case where the deviation is caused at the advance side, and the target phase angle is set to the advance side where the valve overlap between the intake valve and the exhaust valve is originally large, the valve overlap becomes excessive, and the resultant internal EGR quantity becomes excessive, thereby deteriorating the startability of the internal combustion engine.
Further, in the case where the learning of the retention duty value has not yet been completed, the control of the advance side is limited because the target phase angle is limited. In the internal combustion engine having a valve timing control device that changes the switching timing of the intake valve, in the case where the switching timing is extremely changed to the delay side at the time of starting the internal combustion engine, an intake fuel-air mixture within a combustion chamber is returned into an intake pipe because a close timing of the intake valve is delayed. When the intake fuel-air mixture is returned into the intake pipe at the time of cranking where the rotation speed of the internal combustion engine is extremely low, a real compression ratio is lowered to thereby make the startability difficult. In particular, at a low temperature where the volume of the fuel-air mixture is small, there arises such a problem that the fuel-air mixture is not sufficiently compressed even if cranking is conducted, and the startability is further deteriorated.